1. Field of the Invention
This invention relates to a method of storing H2 in metal-doped carbon-based solid state sorbents, such as carbon nanotubes (including stacked truncated carbon nanocones), carbon nanofibers, activated carbon, carbon fibers, graphite and amorphous carbon. The present invention also relates to metal-doped carbon-based sorbent materials capable of absorbing up to 25 wt % of hydrogen at moderate temperature and pressure.
2. Description of Related Art
Hydrogen has been recognized as an ideal energy carrier. However, end-user hydrogen storage is still one of the challenging technical problems attracting increasing research interest1-5 to make it truly useful. In fact, a substantial number of research groups worldwide have put intensive effort to try to use a hydrogen based fuel cell as a power source for automobiles and other devices. These groups have encountered problems associated with high cost and low efficiency of the hydrogen storage systems.
Currently, there are four kinds of H2 storage systems in use: (a) liquid hydrogen, (b) compressed gas, (c) cryo-adsorption and (d) metal hydride storage systems. A brief description of these existing methods is given below:
(a) The liquid hydrogen storage system is of importance because it offers good solutions in terms of technology and economy, both for mobile storage systems and large-volume storage for volumes of from 100 liters to 5000 m3. However, in order to store the liquefied hydrogen, the container (dewar) should be made of super-insulating materials, which is very expensive in practice.
(b) The compressed gas storage system is usually applied in underground supply systems, similar to a network of natural gas. This is an economical and simple method, but it is unsafe and not portable.
(c) Cryo-adsorbing storage systems show advantages in moderate weight and volume. In this system, hydrogen molecules are bound to the sorbent only by physical adsorption forces, and remain in the gaseous state. The adsorbing temperature is in the range of 60 to 100 K. Normally, activated carbon is used as the sorbent due to its large portion of small pores serving as arenas for storing H2. The efficiency of H2 uptake is no more than 7 wt %, which is equivalent to about 20 kg H2 per cubic meter of activated carbon. The disadvantages of this system relate to the low capacity and the much lower temperature required, which, similar to that of liquid hydrogen systems, makes it necessary to use suitable super-insulated containers, and thus the cost is increased.
(d) Metal hydride storage systems are recognized as a novel concept in hydrogen storage. These store large quantities of H2 via a chemical reaction of H+M=Mxe2x80x94H, wherein M is a selected metal system. Two major metal systems, i. e. Fexe2x80x94Ti and Mgxe2x80x94Ni, have been applied as H2 storage media and have been put into use in automobiles driven by an H2/O2 fuel cell. The operating temperature is 40-70xc2x0 C. for the Tixe2x80x94Fe system and 250-350xc2x0 C. for the Mgxe2x80x94Ni system. The H2 storage capacity is less than 5 wt % for Nixe2x80x94Mg and 2 wt % for Fexe2x80x94Ti, which corresponds to less than 70 kg H2 per cubic meter of metals. Furthermore, metal hydride systems normally require 20-40 bar pressure to keep the hydrogen in equilibrium. This renders the container for the metal hydride too heavy and expensive, and limits the practical exploitation of these systems.
The last two mentioned H2 storage systems above are chosen only in some special applications due to their relatively low H2 storage capacity and high cost. Embodiments of the present invention advantageously provide systems which increase H2 storage capacity relative to prior systems and also provide for hydrogen storage under practical conditions.
Alkali-metal based materials have been reported as being able to absorb H2. For Li-based materials, normally in the case of an Li battery, hydrogen absorption takes place under electro-chemical conditions1. LiH can be formed at 300-500xc2x0 C., but the dissociation requires 700-900xc2x0 C. For K-based materials, it has been reported that about 160 ml of hydrogen was absorbed by 1 gram K-intercalated graphite at liquid nitrogen temperature6. The intercalation of alkali metal into graphene layers may form a compound, CnM.
More recently, several articles have been published7-8 concerning storing H2 on carbon materials . A. C Dillon et al. reported that about 0.01 wt % of H2 was absorbed by raw carbon nanotube material (which was estimated to constitute 5 wt % of the single wall nanotube material) at 130 K, and A. Chambers et al. reported 65 wt % of hydrogen uptake was achieved by using herringbone-like graphite nanofibers under 200 atm pressure.
The present invention advantageously provides a method which increases the hydrogen storage capacity of a solid sorbent. Advantageously, the present invention also provides a method which enables the storage of hydrogen to be reversibly performed under ambient or higher pressure and moderate temperature. Still further, the present invention advantageously provides a means to economically make an efficient sorbent.
The above advantages may be achieved by modifying the nature of a carbon-based material which is to be used as the sorbent in a hydrogen storage system. The modification according to the invention comprises the doping of the carbon-based material with a metal which doping causes a distinct change in the structural and electronic properties of the carbon-based material.
The terms xe2x80x9cdopingxe2x80x9d and xe2x80x9cdopedxe2x80x9d, in the context of the description of the present invention, refers to the addition of metal to carbon materials with the result that the structural and electronic properties of the carbon material are changed. This contrasts with a more strict definition of doping where actual replacement of carbon atoms within the graphitic structure is assumed. Without being bound by any theory of the invention, the inventors believe that a carbon metal compound structure is formed.
According to one aspect of the invention, there is provided a method of reversibly storing hydrogen comprising exposing a solid sorbent of metal-doped carbon-based material to a hydrogen atmosphere at a temperature of from about 250 K to about 973 K under ambient or higher H2 pressure, preferably from about 1 to about 200 atm, more preferably from about 1 to about 100 atm and most preferably from about 1 to about 5 atm.
According to another aspect of the invention, there is provided a method of reversibly storing hydrogen comprising pre-treatment of a solid sorbent comprising a metal-doped carbon-based material in an inert atmosphere at high temperature before exposing a solid sorbent of metal-doped carbon-based material to a hydrogen atmosphere at a lower temperature under ambient or higher H2 pressure.
According to another aspect of the invention, there is provided a method of preparing an alkali metal-doped carbon-based material for use in reversibly storing hydrogen comprising mixing a carbon material with an alkali metal salt and calcining the mixture under an atmosphere of inert or reductive gases.
There is also provided a hydrogen storage system comprising an alkali metal-doped carbon-based material prepared in accordance with the method described in the immediately preceding paragraph.
Additional information concerning this invention is contained in P. Chen et al., Science 285 91 (1999), the entire contents of which are hereby incorporated by reference.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. These variations are considered to be in the scope of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.